The invention relates to gas turbines and more particularly to the configuration and the assembly of an annular combustion chamber having inner and outer walls made of ceramic matrix composite (CMC) materials. The fields of application of the invention comprise gas turbine aero-engines and industrial gas turbines.
Proposals have been made to use CMCs for making gas turbine combustion chamber walls because of the thermostructural properties of CMCs, i.e. because of their ability to conserve good mechanical properties at high temperatures. Higher combustion temperatures are sought in order to improve efficiency and reduce the emission of polluting species, in particular for gas turbine aero-engines, by reducing the flow rate of air used for cooling the walls. The combustion chamber is mounted between inner and outer metal casings by means of link elements that are flexible, i.e. elements that are elastically deformable, thus making it possible to absorb the differential dimensional variations of thermal origin that occur between metal portions and CMC portions. Reference can be made in particular to documents U.S. Pat. Nos. 6,708,495 and 7,234,306.
CMC materials are constituted by refractory fiber reinforcement, e.g. made of carbon fibers or of ceramic fibers, which reinforcement is densified by a ceramic matrix. In order to make a CMC part of complex shape, a fiber preform is prepared of shape that is close to the shape of the part that is to be made, and then the preform is densified. Densification may be performed by a liquid process or by a gas process, or by a combination of both. The liquid process consists in impregnating the preform with a liquid composition that contains a precursor for the ceramic matrix that is to be made, the precursor typically being a resin in solution, and then pyrolytic heat treatment is performed after the resin has been cured. The gas process is chemical vapor infiltration (CVI), which consists in placing the preform in an oven into which a reaction gas phase is introduced to diffuse within the preform and, under predetermined conditions, in particular of temperature and pressure, to form a solid ceramic deposit on the fibers by decomposition of a ceramic precursor contained in the gas phase or by a reaction occurring between components of the gas phase.
Whatever the densification process used, tooling is required to hold the preform in the desired shape, at least during an initial stage of densification for consolidating the preform.
Making annular combustion chamber walls for a gas turbine requires tooling that is complex in shape. Furthermore, when performing densification by CVI, preforms can occupy a large amount of space in a densification oven, and it is highly desirable to optimize the way in which the oven is loaded.
Document U.S. Pat. No. 4,907,411 proposes a combustion chamber in which the walls are subdivided longitudinally and circumferentially into ceramic panels. At their longitudinal ends, the panels are folded back to form channel-section portions and they are supported by annular metal parts that are fastened to an outer metal casing and that engage in the channel-section portions with insulating elements being interposed between them.
Document U.S. Pat. No. 3,956,886 shows combustion chamber walls made in the form of ceramic tiles. The tiles may present folded edges of channel section that are inserted in housings defined by metal parts fastened to inner and outer metal casings, thereby making the assembly somewhat inappropriate in the event of differential expansion.
Document EP 1 635 118 proposes using CMC tiles to make a chamber wall that is exposed to hot gas, which tiles are supported by a support structure that is spaced apart from the chamber wall. The tiles are formed with tabs that extend into the space between the chamber wall and the support structure and that extend through the support structure so as to be connected thereto on the outside. The connections are rigid and occupy significant volume outside the support structure. In addition, the presence of an additional casing is required in order to provide sealing.